This invention relates to negative electrodes and their preparation. It particularly relates to methods of preparing electrodes which employ solid lithium-aluminum Alloys as active material. Electrochemical cells using electrodes of this type are being developed to provide high current and capacity densities, high specific power and long cycle life. Such cells have application in batteries for powering an electric vehicle or for the storage of electric power generated by electric utilities during periods of off-peak usage.
A substantial amount of work has been done in the development of electrodes that employ solid alloys of alkali metals or alkaline earth metals. Of these, lithium and calcium alloys have received the most attention as negative electrode active materials. Secondary electrochemical cells having high specific energy and capacity can be prepared from these negative electrodes opposite to positive electrodes includng chalcogens or metal chalcogenides as active materials. For example, sulfides of the transition metals are particularly well suited as positive electrode reactants.
Examples of such secondary electrochemical cells and their various components are disclosed in U.S. Pat. Nos. 3,716,409 to Cairns et al., entitled "Cathods for Secondary Electrochemical Power-Producing Cells", Feb. 13, 1973; 3,887,396 to Walsh et al., June 3, 1975, entitled "Modular Electrochemical Cell"; and 3,907,589 to Gay et al., Sept. 23, 1975, entitled "Cathode for a Secondary Electrochemical Cell". In addition, various methods of preparing electrodes are illustrated in U.S. patent application Ser. Nos. 481,285 to Settle et al., filed June 20, 1974 now U.S. Pat. No. 3,957,532, May 18, 1976, and 565,021 to Gay et al., filed Apr. 3, 1975 now U.S. Pat. No. 3,933,520, Jan. 20, 1976. Each of these patents and patent applications are assigned to the assignee of the present application.
Negative electrodes employing lithium-aluminum active material have previously been prepared either by electrochemical processes or by alloying the two elements at temperatures above the liquidus. In electrochemically depositing lithium metal into an aluminum substrate, a number of lengthy electrochemical cycles are often required. Care must be taken in the cycling to maintain uniform deposition and sufficient porosity within the substrate.
Lithium-aluminum alloys prepared from molten mixtures of the two elements have been solidified and annealed to enhance uniformity. The resulting solid alloys were comminuted to particles and either vibratorily loaded into an electrically conductive, porous structure or hot-pressed with molten electrolyte into moldes containing current collector structures to form electrodes. Handling active material in powdered form involves the risk of contamination. These techniques also are difficult to adapt to large production rates, as they require close control to attain uniformity in porosity and distribution of active materials. The electrodes undergo redistribution or slumping of the active materials during operation within an electrochemical cell.
The inventors, being aware of these prior art problems, have as objects of their invention an uncomplicated method that can be performed in a short time for preparing porous lithium alloy electrodes with electrically conductive current collectors.
It is also an object to provide a method that permits easy control of electrode porosity and active material loading.
It is a further object to provide an improved negative electrode containing a solid lithium alloy for use within a secondary electrochemical cell.